Signal repeater and re-driver circuits are used in networking, communication, computer, and a variety of other systems. Upstream chips may have a limited drive capability, limiting the distance that signals may be safely driven. Standards such as Peripheral Components Interconnect Express (PCIE) and others may specify a limit to the length of a signal line. A long distance line may be divided into two or more segments, with a buffer or repeater between segments.
A buffer chip may be used to receive the signal from a first segment, regenerate the signal, and output the signal to the next segment. A wide variety of simple buffer chips are available. However, modern signals are often differential, small-voltage-swing signals. Buffers must not distort the signal's characteristics. Thus specialized differential buffers may be needed that are very sensitive to small-swing signals.
Signal lines are often provided with termination to improve signal characteristics. For example, the far end of a transmission line that is near the receive may have a resistor to ground, with the near end of the transmission line may have a termination resistor to power. The two resistors set the voltage of the transmission line to an intermediate voltage between power and ground when no signals are being transmitted. The termination resistors absorb injected noise on the transmission line and reduce or prevent large voltage swings from occurring. The A.C. characteristics of the transmission line at a desired signaling frequency may be enhanced by tuning the values of the termination. The exact values of termination resistors may be determined empirically by soldering different-value termination resistors into a test board and selecting the resistor value that produces the best characteristics.
One problem with re-driver circuits is that the termination may change during operation. At times, some lanes of differential signals may be powered-down and their termination disconnected to save power. A simple buffer chip used as a re-driver that is inserted between a pair of line segments may block the chip that transmits to the first segment from seeing the change in termination of the last segment. The transmitting chip may be prevented from powering down by the buffer chip.
What is desired is a re-driver integrated circuit that is designed for repeating differential small-swing signals. A re-driver that senses far-end termination on a next segment that is driven by the redriver is desirable. A re-driver chip that senses far-end termination on its output and mirrors that termination back to its input is desirable. A re-driver chip that also shapes, amplifies, and equalizes signals to improve signal characteristics is also desirable.